cAWJweJw*te\
;
,
'
/
1
'
//
^ii4Ugrt*jisi/j«7^->
\J
ii
r
&& JAJL
/.;
j0^c4+4&-4
OrganocatalytU Depolymerization of Poly(ethylene terephthalate)
KAZUKI FUKUSHIMA,1 OUVIER COULEMBIER,2 JULIEN M. LECUYER,1 3 HAMID A. ALMEGREN,*
ABDULLAH M. ALABDULRAHMAN,4 FARES D. ALSEWAILEM,* MELANIE A MCNEIL.* PHILIPPE DUBOIS,2
ROBERT M. WAYMOUTH.6 HANS W. HORN.' JULIA E. RICE,1 JAMES L. HEDRICK1
'IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120
laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials
and Polymer (CIRMAP), University of Mons, Place du Pare 23, B-7000 Mons, Belgium
Department of Chemical and Materials Engineering, San Jose State University, San Jose, California 95192
"Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia
5Department
of Chemistry, Stanford University, Stanford, California 94305
Received 4 November 2010; accepted 16 December 2010
DOI: 10.1002/po!a.24551
Published online 11 January 2011 in Wiley Online Library (witeyonlinelibrary.com).
ABSTRACT: We describe the organocatalytic depolymerization
of polylethylene terephthalate) (PET), using a commercially
available guanidine ca yst, 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD). Postconsumer ET beverage bottles were used and
processed with 1.0 n I % (0.7 wt %) of TBD and excess
amount of ethylene gly ol (EG) at 190 °C for 3.5 hours under
atmospheric pressure t give bis(2-hydroxyethyl) terephthalate
(BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/a I koxide catalysts that are commonly used for depolymerization of PET. The BHET content in
the glycolysis product was subject to the reagent loading. This
INTRODUCTION Advances in technology continue to present
many environmental issues making waste management a
significant challenge. Landfill space is at a premium, even if
the total amount of municipal solid waste (MSW) going to
landfills in US has dropped since 1990. The plastic refuse
generated in US constitutes 12% of the MSW in 2008; while
relatively modest as a percentage, plastic waste is the 4th
major component of the MSW after paper, food, and yard
trimmings. 1 Poly(ethylene terephthalate) (PET), a widely
used commodity-grade thermoplastic contributes several
billion pounds of waste to landfills every year, and the
amount of PET needed is unlikely to diminish any time
soon.2 Recycling of petroleum-based plastics has recently
attracted enormous attention to promote effective use of limited fossil resources to mitigate impacts on solid waste.
According to the American Plastics Council, now only 27%
of the PET bottles and jars are recycled and the PET market
for packaging continues to grow due to the popularity of
PET-packaged products, such as bottled water.3
The challenge for PET recycling is to achieve a closed-loop,
bottle-to-bottle process, similar to the aluminum cans (48%
catalyst influenced the rate of the depolymerization as well as
the effective process temperature. We also demonstrated the
recycling of the catalyst and the excess EG for more than
5 cycles. Computational and experimental studies showed
that both TBD and EG activate PET through hydrogen bond
formation/activation to facilitate this reaction. £> 2011 Wiley
Periodicals, Inc. J Potym Sci Part A: Polym Chem 49: 12731281, 2011
KEYWORDS: catalysis; degradation; depolymerization; glycolysis; organocatatyst; polylethylene terephthalate); recycling
recycled).1 Two major conventional methods of recycling
postconsumer PET exist: mechanical recycling and chemical
recycling.4"7 The former is most commonly practiced and
involves sorting, washing and drying postconsumer PET
before melt-process ing to produce a new material. The organometallic catalysts used to synthesize PET such as antimony, titanium or germanium 8 remain permanently in the
fabricated item, leading to significant property deterioration
during the secondary melt fabrication process.9 As a consequence, mechanically-recycled PET generally ends up in secondary products such as fiber for clothing or carpeting, and
engineering resins for reinforced automobile components. 2 10
Ultimately these all find their way to the landfill. The problem was, however, solved by solid state polymerization technique where the catalysts in the waste PET are applied to
increase/maintain the molecular weight high enough for
the fabrication. 11 Mechanical methods for bottle-to-bottle
recycling are being established, but there stilt are some practical concerns especially when colored bottles are used as
raw materials; 12 variation in the amounts and types of residual catalysts in the waste PETs create additional challenges.
zinat Supporting iformation may be found in the online version of this article. Correspondence to: K. Fuki
>rj. E- Rice (E-m il: iulia@almaden.ibm.com) or J. L. Hedrick (E-mail: hedrick@almaden.ibm.com)
i! of Polymer Sci nee Pan A: Polymer Chemistry, Vol. 49, 1273-1281 (2011) C 2011 Wiley Periodicals, Inc.
Chemical recycling entails degradation of the polymer to its
starting monomer, purification, and then subsequent repolymerization to yield high quality plastic.7 Depolymerization
processes for chemical recycling mainly include hydrolysis,
methanolysis, and glycolysis13"15 which are generally conducted at high temperature in the presence of catalysts such
as metal (zinc, lead, cobalt, manganese) acetate, zeolite, titanium(IV) n-butoxide, and sodium/potassium sulfate, and
under the pressure in some cases.15'20 Hydrolysis and methanolysis are more common because the high crystalline
monomers terephthalic acid (TA) and dimethylterephthalate
(DMT) are easier to isolate than the glycolysis product bis(2hydroxyethyl) terephthalate (BHET). In addition, PET is generally prepared by a two step process: the condensation of
TA or DMT with excess ethylene glycol (EG) to generate
BHET followed by the self-condensation of BHET at high
temperatures (270-290 °C) using mixed organometallic catalysts optimized for their reactivity and selectivity for each
step of the process.521 Current processes for the chemical
recycling of PET are energy intensive, and consequently
suffer from unfavorable economics relative to mechanical
recycling, and are therefore not widely practiced.6'7 Low
monomer costs also contribute to the economic challenges
for alternative technologies utilizing postconsumer PET as a
monomer feedstock.22'23 Moreover, the chemical approach to
recycling of PET-based copolyesters 2425 has advantages in
terms of mechanical properties associated with the final
product. It also can be readily extended to other polyesters.26'27 Initiatives in the chemical recycling of PET are
thus ideally focused on developing an environmentally safe,
economically feasible, and industrially applicable process for
wide-scale application. Chemical recycling methodologies
that are energy efficient and do not involve heavy metals are
highly desirable even though the catalysts are usually not
contained in the purified monomers.
Organic catalysts are attractive alternatives to traditional
organometallic reaction promoters. Organic phase transfer
catalysts based on quaternary ammonium salts have been
used for hydrolysis of PET where sodium hydroxide was
used as a cocatalysL14'28 Organocatalysis has been shown to
be a powerful strategy for polymer synthesis. As these catalysts typically operate by different mechanisms than metal
alkoxides, they offer a diversity of mechanistic pathways that
can provide new opportunities for selective polymerization
and depolymerization processes.29 We have developed
several organic catalyst platforms for polymerization and
transesterification reactions.30 l,5,7-Triazabicyclo[4.4.0|dec5-ene (TBD), a potent neutral base (pKa = 26 in acetoni-
trile) 31 well-known as a catalyst for a variety of reactions 32
is among the most active ring-opening polymerization (ROP)
catalysts known. The ROP of lactide with 0.1% TBD in THF
exhibits a turnover frequency of 80 s"1 at room temperature,
a rate comparable to that of the most active metal catalysts
reported for ROP of lactide.33 Computational studies suggest
that TBD is such an effective catalyst because it activates
both alcohol and monomer through hydrogen-bonds.3*'35
The high activity of TBD for transesterification reactions
stimulated us to extend our investigation to depolymerization, rather than polymerization. Herein, we show that
the guanidine TBD is an efficient catalyst for the glycolysis
of PET to its monomer bis(2-hydroxyethyl)terephthalate
(BHET)1637 and also demonstrate its recyclability (Scheme 1).
EXPERIMENTAL
Materials
PET beverage bottles were washed with water, dried, and
shredded to around 3 mm squares prior to use. 1,5,7-triazabicyclo|4.4.0]dec-5-ene (TBD), ethylene glycol (EG; anhydrous, 99.8%), and solvents were used as received (SigmaAJdrich).
Instruments
] H NMR spectra were obtained on a Bruker Avance 400
Instrument at 400 MHz. Size exclusion chromatography
(SEC) was performed in THF at 30 °C using a Waters chromatograph equipped with four 5 ;im Waters columns (300
mm x 7.7 mm) connected in series with increasing pore size
(10, 100, 1000, 105, 106 A), a Waters 410 differential refractometer for refractive index (RI) detection, and calibrated
with polystyrene standards (750 - (2 x 106) g/mol).
Typical Procedure of Glycolysis
To a 25 mL Schlenk tube containing colorless PET flakes
(0.96 g, 5.0 mmol), 38 previously dried at 80 °C for 1 h, was
charged a mixture of EG (5.00 g, 80.6 mmol) and TBD
(70 mg, 0.50 mmol) in a glove box. The tube was immersed
in an oil bath heating at 190 °C to conduct the reaction with
stirring. After 8 minutes the slurry turned into a clear and
homogeneous liquid. The crude solid was purified by either
of the following two methods.
A Extraction: The reaction mixture was cooled to room temperature and dissolved in methylene chloride (100 ml) with
slight heating to dissolve the solid. The solution was washed
with 0.5 N HCI aqueous solution (100 mL) and extracted
with methylene chloride (50 mL). The organic fractions were
combined, stirred over MgS04, evaporated, and dried in
ORGANOCATALYTIC OEPOLYMERIZATION Of PEt, FUKUSHIMA ET AL
Wl LE VON LI NELIBRAR Y.COM/JOURNAL/JPOLA